1. Field of the Invention
The present invention relates to a method for evaluation of renal perfusion, in particular to a method for evaluation of renal perfusion by capturing and analyzing consecutive serial images in renal vessels using power Doppler ultrasonography.
2. The Prior Arts
The main function of kidney is to remove metabolic wastes (mainly nitrogenous wastes) in the body. Declines in renal function may accumulate metabolite wastes in the body and consequently cause multiple-organ dysfunction or even death unless renal dialysis treatment or kidney transplantation is performed. A large amount of blood flow supply is necessary for the renal function, so that 25˜40% cardiac output comes into kidney for the renal perfusion. Therefore the measurement of renal blood flow has become an important tool for the assessment of renal functions. Power Doppler ultrasonography provides a non-invasive method among the various measurement methods of renal perfusion. In the past, renal perfusion examined with power Doppler was explained by a single image from the large renal area with an image of best perfusion. However the clinical application is limited. Because the heart has dynamic blood flow to reach the kidney via aorta. It's difficult to elucidate the dynamic blood flow state with a static image.
Early detection of renal problems is beneficial for proper treatment. During the last decade, both color Doppler and power Doppler sonography have been widely used to evaluate renal functions. Unlike computer tomography (CT) scans and magnetic resonance imaging (MRI), color Doppler images, based on the mean Doppler frequency shift induced by blood flow velocity, easily present the overall renal vascular structure, as well as the flow direction and velocity. However, color Doppler is not sensitive in depicting small vessels or vessels with low velocity. Power Doppler sonography is based on the total integrated energy of Doppler shifted echoes and has higher sensitivity for much slower blood flow, but it can not address the flow direction or flow velocity. Lately, a directional power Doppler imaging mode has been implemented in commercially available duplex ultrasound systems in which the positive and negative flows are separated before the estimation of Doppler strength. The flow direction is encoded in the power Doppler image with a two-color scale which showed some clinical applications.
Pulsed Doppler ultrasound clinically measures vascular resistive index (RI), pulsatility index (PI) and systole/diastole ratio through blood flow velocity, which shows the renal vascular resistance and is often used to estimate the changes of flow velocity. The traditional S/D ratio, RI or PI measured using pulse Doppler imaging can only depict the flow at the interlobar vessels and at most down to the size of arcuate vessels. In addition, these methods showed poor specificity and sensitivity.
Previously, Power Doppler was applied to examine vascular structure of the renal cortex (areas covered by arcuate vessels and interlobular vessels), such as to detect renal vascular perfusion in pigs after administration of adrenocorticotropic hormone and papaverine. Power Doppler was also applied to detect impaired perfusion in acute pyelonephritis, and to identify cases of acute or chronic vascular rejection with interlobular vascular signal after renal transplantation. However, all these applications collected “static” information on renal perfusion without details on haemodynamics, which were not sufficient to reflect the actual dynamic condition of renal perfusion.